In recent years, the amounts of traffics generated in networks are continuously increasing due to popularization of smart phones, high-resolution TVs and the like, and transmission speeds for channels of optical transmission networks are expected to increase up to 400 Gb/s or 1 Tb/s, exceeding 100 Gb/s. A non-return-to-zero (NRZ) method where only existing magnitudes are turned on and off through a modulation method of generating high-speed signals and transmitting the high-speed signals through optical links has a limit due to a limit in bandwidths of devices and high optical signal-to-noise ratio (OSNR) requirements. Thus, a quaternary phase shift keying (QPSK) method where the number of symbols transmitted per bit is increased while modulating the phase of an optical signal or a quadrature amplitude modulation (QAM) method where the magnitude and phase of an optical signal are simultaneously modulated are being introduced.
As a transmission speed of an optical signal increases or the number of bits transmitted per symbol increases, a higher optical signal-to-noise ratio (OSNR) is required. In general, since OSNR becomes lower as a distance from an optical transmission line is longer, a signal having a high transmission speed and having a large number of bits transmitted per symbol has a short maximum transmission distance by which the signal can be transmitted.
In general, an optical transmitter is tuned to a fixed frequency defined by ITU-T Standards, and an interval between frequencies is 50 GHz as illustrated in FIG. 1A (Fixed frequency grid). However, if a transmission speed per one channel of an optical signal reaches 400 Gb/s, exceeding 100 Gb/s, the signal cannot be accommodated in the fixed frequency interval defined by ITU-T Standards. It is because a bandwidth occupied by a signal becomes larger than 50 GHz due to a high transmission speed and thus an interference with a neighboring channel may occur. In order to avoid such an interference, a method where a guard band is present between a neighboring channel and a 400 Gb/s signal may be used but spectral efficiency is reduced. Thus, a flexible frequency grid where an interval of frequency grids is subdivided into 12.5 GHz to accommodate a 400 Gb/s signal in an optical transmission network and prevents waste of frequencies is being introduced as a new method. FIG. 1B illustrates transmission spectrums of a flexible frequency grid. In the flexible frequency grid, a wavelength of an optical transmitter is located at arbitrary frequencies disposed at an interval of 12.5 GHz and an occupied bandwidth is determined according to the number of slots having a frequency interval of 12.5 GHz. Although a flexible frequency grid for freely changing a center frequency and a bandwidth of an optical signal is required to accommodate a 400 Gb/s optical signal together with an existing 40 Gb/s or 100 Gb/s optical signal in an optical transmission network, a 400 Gb/s cannot be accommodated in an existing fixed frequency grid.